Growth Mechanism of Periodic-Structured MoS2 by Transmission Electron Microscopy

Molybdenum disulfide (MoS2) was grown on a laser-processed periodic-hole sapphire substrate through chemical vapor deposition. The main purpose was to investigate the mechanism of MoS2 growth in substrate with a periodic structure. By controlling the amount and position of the precursor, adjusting the growth temperature and time, and setting the flow rate of argon gas, MoS2 grew in the region of the periodic holes. A series of various growth layer analyses of MoS2 were then confirmed by Raman spectroscopy, photoluminescence spectroscopy, and atomic force microscopy. Finally, the growth mechanism was studied by transmission electron microscopy (TEM). The experimental results show that in the appropriate environment, MoS2 can be successfully grown on substrate with periodic holes, and the number of growth layers can be determined through measurements. By observing the growth mechanism, composition analysis, and selected area electron diffraction diagram by TEM, we comprehensively understand the growth phenomenon. The results of this research can serve as a reference for the large-scale periodic growth of MoS2. The production of periodic structures by laser drilling is advantageous, as it is relatively simpler than other methods.


Introduction
One of the most important two-dimensional (2D) transition metal chalcogenides that is gaining increased attention is MoS 2 [1][2][3][4][5][6][7]. Materials with nanoscale electronic and optoelectronic components, such as field-effect transistors, prospective memory components, light-emitting diodes, and sensors, have been manufactured by exploiting the excellent spinvalley coupling and flexural and optoelectronic properties of MoS 2 [8][9][10][11][12][13][14][15][16][17][18][19][20]. Two-dimensional MoS 2 is low cost and does not require complex preparation [21]. To date, MoS 2 -based semiconductor heterostructures, such as CdS/MoS 2 , MoO 3 /MoS 2 , and SnO 2 /MoS 2 , featuring good photocatalytic or photoelectrochemical properties, have been successfully synthesized owing to the efficient charge separation obtained by coupling two semiconductor structures with matched energy levels [22][23][24][25]. Nevertheless, the mass production of such devices demands a method of synthesizing large-scale, layer-controlled, high-quality MoS 2 . Most studies on MoS 2 films with excellent results have been obtained using a top-down approach, such as mechanical exfoliation [26,27]. Other studies are moving from characterizing 2D thin films to manufacturing low-cost devices, mass producing logic-integrated circuits, and growing 2D materials on foreign substrates. They aim to replace the existing exfoliation and liquid exfoliation methods for producing randomly distributed flakes and providing limited control of the number of MoS 2 film layers. Chemical vapor deposition (CVD) is the usual vapor-phase growth method used to create semiconductor thin films and heterojunctions [28]. CVD is gaining increased attention owing to its success in the growth of large-area, high-quality, and uniform nanofilms [29][30][31][32][33]. In the present work, MoS 2 was grown on a laser-processed periodic-hole sapphire substrate through the CVD method. We aimed to analyze the growth mechanism of MoS 2 with a regular structure by controlling the amount and location of the precursor, modifying the growth temperature and time, and setting the flow rate of argon gas (Ar) to induce MoS 2 growth around the periodic holes.

Preparation of Laser Drilling for Substrates Containing Periodic Structures
The most direct way to prepare many periodic patterned microstructures is to prepare traces on the substrate surface and to use laser processing to drill holes with about 10 µm diameter and 300 nm depth. Laser drilling damages the substrate structure and causes unevenness around the hole, which has a certain degree of influence on the MoS 2 growth mechanism.

Growth of MoS 2 on Sapphire Substrate by CVD
The substrate used to grow MoS 2 was sapphire with silicon dioxide (SiO 2 ) on the surface (see Supplementary Materials Section 1 for obtaining the MoS 2 layers). Periodic holes were made by laser processing, and then CVD was performed. The precursors used were sulfur powder (S) with a purity of 99.98% and molybdenum oxide powder (MoO 3 ) with a purity of 99.95%. High-purity chemical powders were used to remove impurities remaining in the experimental cavity and affecting the CVD. It was also possible to grow single crystals with residual impurities. The chemical solvent used to clean the remaining chemical substances after each experiment was aqua regia. The concentrations of nitric acid and hydrochloric acid (HCl) used to prepare aqua regia were 37 vol.% and 68-69 vol.%, respectively. The quartz tube and ceramic crucible were cleaned inside the tubular thermal furnace with this solution (see Supplementary Materials Section 2.6 for tube furnaces). The quartz tube was regularly replaced according to the change in residual sulfur powder on the tube wall. The purpose was to reduce the impact of experimental environmental factors. The substrates, organic solvents, gases, and chemicals used are detailed in Table 1. Before the start of the experiment, the cleanliness of the substrate was confirmed (Supplementary Materials Section 2). First, 1 g of sulfur and 0.003 g of MoO 3 were prepared and placed in appropriate positions in the inner tube of the quartz tube. The substrate was set inverted on molybdenum trioxide (MoO 3 ). The inert gas used during growth was Ar. Initially, 500 sccm of Ar was let in for 10 min to clean the internal cavity. More water and oxygen were removed in the quartz tube, after which, the Ar flow was reduced to 200 sccm. The heating rate was set to 20 • C/min. The maximum temperature was set to 700 • C, and the heating time was set to 35 min. After reaching the maximum temperature, the same was maintained for 50 min. Finally, once the temperature dropped to <400 • C, it was further dropped to room temperature by opening the lid, and the MoS 2 structure was obtained. The experimental process is shown in Figure 1.
was further dropped to room temperature by opening the lid, and the MoS2 structure was obtained. The experimental process is shown in Figure 1. Considering that the vaporization point of MoO3 is above 650 °C and the vaporization point of S is above 200 °C, MoO3 in the gas phase underwent two chemical reactions in high-temperature environments to produce the intermediate molybdenum oxide (MoO3−x). This intermediate molybdenum oxide diffused to the substrate and reacted with vaporized sulfur to form MoS2 film. The distance between the two crucibles containing the precursor was 46 cm. Single-layer and multilayer MoS2 can be effectively prepared by CVD, as shown in the following equations:

Growth Mechanism of MoS2
The properties of MoS2 are useful for semiconductors and optoelectronic materials with single or few layers. CVD is the most commonly used method for MoS2 growth. Given that the location of CVD growth of single-layer MoS2 is relatively random, understanding its growth mechanism can substantially benefit research on MoS2 growth. TEM helps elucidate the atomic structure and the chemical composition information of particle evolution during catalysis [34][35][36][37][38][39][40]. The material properties of MoS2 are preferably a single layer or very few layers. The analysis of the number of layers depended on the Raman spectra, atomic force microscopy (AFM) images, and photoluminescence (PL) spectra (see Supplementary Materials Section 2.1 for micro-Raman spectroscopy; see Supplementary Materials Section 2.2 for micro-Raman micro-PL spectrometer). Raman measurements were made using a laser with a wavelength of 532 nm as the excitation light source. TEM can also be used to measure the number of observation layers of the substrate cross-section (see Supplementary Materials Section 2.7 for transmission electron microscope). The multilayer structure in the cross-section geometry was studied by TEM using lamella specimens produced by focused ion beam (FIB) milling. The phase state of the multilayer volume was assessed by selected area electron diffraction (SAED) pattern analyses and fast Fourier transform (FFT) patterns generated from the corresponding regions of the HRTEM images (see Supplementary Materials Section 3 for TEM analysis). The properties of MoS 2 are useful for semiconductors and optoelectronic materials with single or few layers. CVD is the most commonly used method for MoS 2 growth. Given that the location of CVD growth of single-layer MoS 2 is relatively random, understanding its growth mechanism can substantially benefit research on MoS 2 growth. TEM helps elucidate the atomic structure and the chemical composition information of particle evolution during catalysis [34][35][36][37][38][39][40]. The material properties of MoS 2 are preferably a single layer or very few layers. The analysis of the number of layers depended on the Raman spectra, atomic force microscopy (AFM) images, and photoluminescence (PL) spectra (see Supplementary Materials Section 2.1 for micro-Raman spectroscopy; see Supplementary Materials Section 2.2 for micro-Raman micro-PL spectrometer). Raman measurements were made using a laser with a wavelength of 532 nm as the excitation light source. TEM can also be used to measure the number of observation layers of the substrate cross-section (see Supplementary Materials Section 2.7 for transmission electron microscope). The multilayer structure in the cross-section geometry was studied by TEM using lamella specimens produced by focused ion beam (FIB) milling. The phase state of the multilayer volume was assessed by selected area electron diffraction (SAED) pattern analyses and fast Fourier transform (FFT) patterns generated from the corresponding regions of the HRTEM images (see Supplementary Materials Section 3 for TEM analysis).

CVD Growth of Periodic MoS2
The CVD method is used to grow MoS2, and a substrate with periodic holes is prepared by laser drilling in a high-temperature furnace tube to grow MoS2.

Image Analysis under Optical Microscopy (OM)
Many methods are well established to identify the number of MOS2 layers, but in this study, an optical microscope was used (Supplementary Materials Section 2.3, optical microscope) [41][42][43][44][45][46]. OM reveals the growth pattern of MoS2. Figure 2 shows the OM and SEM images of MoS2 grown on a sapphire substrate.   Figure 3c is the OM image at position c in Figure 3a, where MoS2 grows regularly in the hole area, with a bright blue ring pattern on the edge of the hole. This finding shows the existence of a multilayer MoS2. The light blue irregular shape distribution on the periphery of the hole is represented by the existence of single-layer MoS2. Figure 3d is the OM image at position d in Figure 3a, where we can see that a single layer of MoS2 grows along the hole periphery. The periodic growth is not as good as that at the c position, but the single-layer MoS2 covers a large area with no highlevel MoS2. Figure 3e is the OM image at position e in Figure 3a, where we can see that the single layer of MoS2 grows in a more broken manner on the substrate and covers a larger area, but no MoS2 exists around the hole. These results show that according to the Ar flow direction, MoS2 grows in the order of large to small coverage area and from a broken and to a more regular growth around the hole until it no longer exists. Thus, we infer that the MoS2 growth is cyclical.   Figure 3c is the OM image at position c in Figure 3a, where MoS 2 grows regularly in the hole area, with a bright blue ring pattern on the edge of the hole. This finding shows the existence of a multilayer MoS 2 . The light blue irregular shape distribution on the periphery of the hole is represented by the existence of single-layer MoS 2 . Figure 3d is the OM image at position d in Figure 3a, where we can see that a single layer of MoS 2 grows along the hole periphery. The periodic growth is not as good as that at the c position, but the single-layer MoS 2 covers a large area with no high-level MoS 2 . Figure 3e is the OM image at position e in Figure 3a, where we can see that the single layer of MoS 2 grows in a more broken manner on the substrate and covers a larger area, but no MoS 2 exists around the hole. These results show that according to the Ar flow direction, MoS 2 grows in the order of large to small coverage area and from a broken and to a more regular growth around the hole until it no longer exists. Thus, we infer that the MoS 2 growth is cyclical.

Raman Spectrum Analysis Results
Raman mapping, which is the best method for analyzing the number of layers, is used to analyze the image [47][48][49][50]. The results reveal that MoS2 has two peaks at 380 and 400 cm −1 , respectively. When the difference (Δk) between the two peaks is less than 20 cm −1 , it is a single-layer MoS2 structure. The peak is attributed to the in-plane (E12g) and out-of-plane (A1g) oscillation modes of MoS2. Figure 4a shows an OM image of MoS2 periodic growth. The black, blue, green, and red boxes indicate the areas where multilayer and one-, two-, and three-layer MoS2 growth are measured, respectively. Figure 4b shows the Raman measurement diagram of each area marked in Figure 5a. The black part demonstrates that the Raman shift is between 384.7 and 408.8 cm −1 , and the value of the peak difference (Δk) is 24.1 cm −1 . This finding indicates more than four layers, which is unsuitable for semiconductor components and optoelectronic components. The blue part shows that the peak difference between the Raman shift of 386.2 and 405.3 cm −1 is 19.1 cm −1 , which represents a single layer. In the green part, we find that the peak difference between the Raman shift of 385.7 and 406.8 cm −1 is 21.1 cm −1 , defined as a two-layer growth of MoS2. In the red part, we find that the peak difference between the Raman shift of 385.2 and 407.3 cm −1 is 22.1 cm −1 , defined as three layers of MoS2. The Raman measurements prove that the periodicity of our growth has one-to three-layer and multilayer characteristics. These findings prove that MoS2, which grows periodically in the hole, has different distributions from single to multiple layers and, thus, has multiple potential applications. The areas of few-layered MoS2 are sufficiently large. Thus, it is highly applicable in semiconductor device manufacturing. Figure 5a shows the result of the PL analysis of the MOS2 sample. Obvious luminescence peaks exist at about 625 and 667 nm, indicating that MoS2 is in a single layer and a few layers, respectively. Each layer corresponds with the valence band spin-orbit coupling splitting of MoS2 direct exciton transition luminescence (B exciton) and direct energy gap recombination luminescence (A exciton). The converted energy is about 1.98 and 1.86 eV, respectively. We infer that this structure is single-layer MoS2. Figure 5b shows the OM image of the selected PL mapping range. The greenish color is due to the light source of the instrument. Figure 5c shows the PL mapping diagram of the selected PL mapping range OM image with a wavelength of 625 nm in Figure 5b. The blue part is the sapphire substrate, and the yellow to red is the distribution from single layer to multilayer. The blue part is the sapphire substrate, and the yellow-orange to red is the distribution from single layer to multilayer. The dashed parts A and F correspond with the grid points of

Raman Spectrum Analysis Results
Raman mapping, which is the best method for analyzing the number of layers, is used to analyze the image [47][48][49][50]. The results reveal that MoS 2 has two peaks at 380 and 400 cm −1 , respectively. When the difference (∆k) between the two peaks is less than 20 cm −1 , it is a single-layer MoS 2 structure. The peak is attributed to the in-plane (E 12g ) and out-of-plane (A 1g ) oscillation modes of MoS 2 . Figure 4a shows an OM image of MoS 2 periodic growth. The black, blue, green, and red boxes indicate the areas where multilayer and one-, two-, and three-layer MoS 2 growth are measured, respectively. Figure 4b shows the Raman measurement diagram of each area marked in Figure 5a. The black part demonstrates that the Raman shift is between 384.7 and 408.8 cm −1 , and the value of the peak difference (∆k) is 24.1 cm −1 . This finding indicates more than four layers, which is unsuitable for semiconductor components and optoelectronic components. The blue part shows that the peak difference between the Raman shift of 386.2 and 405.3 cm −1 is 19.1 cm −1 , which represents a single layer. In the green part, we find that the peak difference between the Raman shift of 385.7 and 406.8 cm −1 is 21.1 cm −1 , defined as a two-layer growth of MoS 2 . In the red part, we find that the peak difference between the Raman shift of 385.2 and 407.3 cm −1 is 22.1 cm −1 , defined as three layers of MoS 2 . The Raman measurements prove that the periodicity of our growth has oneto three-layer and multilayer characteristics. These findings prove that MoS 2 , which grows periodically in the hole, has different distributions from single to multiple layers and, thus, has multiple potential applications. The areas of few-layered MoS 2 are sufficiently large. Thus, it is highly applicable in semiconductor device manufacturing. Figure 5a shows the result of the PL analysis of the MOS 2 sample. Obvious luminescence peaks exist at about 625 and 667 nm, indicating that MoS 2 is in a single layer and a few layers, respectively. Each layer corresponds with the valence band spin-orbit coupling splitting of MoS 2 direct exciton transition luminescence (B exciton) and direct energy gap recombination luminescence (A exciton). The converted energy is about 1.98 and 1.86 eV, respectively. We infer that this structure is single-layer MoS 2 . Figure 5b shows the OM image of the selected PL mapping range. The greenish color is due to the light source of the instrument. Figure 5c shows the PL mapping diagram of the selected PL mapping range OM image with a wavelength of 625 nm in Figure 5b. The blue part is the sapphire substrate, and the yellow to red is the distribution from single layer to multilayer. The blue part is the sapphire substrate, and the yellow-orange to red is the distribution from single layer to multilayer. The dashed parts A and F correspond with the grid points of the X-axis position of the different color curves in Figure 5d. The color line segments A-F in the upper right corner represent the number of grids selected in the X-axis direction of the mapping grid number 37 × 37 in Figure 5d. The positions of the line segments with higher strength are A, B, and C. A higher number of layers of growing MoS 2 corresponds with decreased strength, as shown in positions D, E, and F. From the results of the PL spectrum analysis, we infer from the mapping image presented by the excitation light peaks at 625 and 667 nm that the periodic growth of MoS 2 has good uniformity in the distribution of monolayer to multilayer.

Selected Area Electron Diffraction
From the composition analysis, we can infer the mechanism of the periodic growth of MoS2. We use the additional function of the TEM system to convert the diffraction image of the selected area into a lattice arrangement through FFT. The red line in Figure 6a indicates the FIB sampling position (see Supplementary Materials Section 2.5 for dualbeam FIB). Figure 6g is the SAED image of Figure 6d, with a miller index of [001]. In this direction, we can see the multilayer-structured MoS2 lattice array. The distance between the layers reveals the hexagonal crystal structure of MoS2 with lattice constants a = 0.318 nm and c = 1.299 nm. Moreover, the crystal plane distance between the multilayer MoS2 layer is 6.2 A, which is close to the 2-H MoS2 crystal plane distance of about 6.5 A. Figure  6h is the SAED image of Figure 6e. The miller index is [001], which shows that the lattice arrangement is chaotic, but the faint lattice points in the four directions may be MoO3. Figure 6i is the SAED image of Figure 6f. The miller index is still [001], and the selected area is the junction of the mixed area and the sapphire substrate at the obvious double layer of HRTEM. Given that the boundary may diffract from the lattice of the upper and lower components of the boundary, it is more mixed. However, from the center point, the miller index [001] can be found, and the distance between the crystal planes is calculated to be about 6.4 Å.

Selected Area Electron Diffraction
From the composition analysis, we can infer the mechanism of the periodic growth of MoS 2 . We use the additional function of the TEM system to convert the diffraction image of the selected area into a lattice arrangement through FFT. The red line in Figure 6a indicates the FIB sampling position (see Supplementary Materials Section 2.5 for dual-beam FIB). Figure 6g is the SAED image of Figure 6d, with a miller index of [001]. In this direction, we can see the multilayer-structured MoS 2 lattice array. The distance between the layers reveals the hexagonal crystal structure of MoS 2 with lattice constants a = 0.318 nm and c = 1.299 nm. Moreover, the crystal plane distance between the multilayer MoS 2 layer is 6.2 A, which is close to the 2-H MoS 2 crystal plane distance of about 6.5 A. Figure 6h is the SAED image of Figure 6e. The miller index is [001], which shows that the lattice arrangement is chaotic, but the faint lattice points in the four directions may be MoO 3 . Figure 6i is the SAED image of Figure 6f. The miller index is still [001], and the selected area is the junction of the mixed area and the sapphire substrate at the obvious double layer of HRTEM. Given that the boundary may diffract from the lattice of the upper and lower components of the boundary, it is more mixed. However, from the center point, the miller index [001] can be found, and the distance between the crystal planes is calculated to be about 6.4 Å.  Sample 2 is analyzed using the TEM image defined in Figure 7. The sampling position does not pass through the hole. The growth of MoS2 on the surface is analyzed, as shown in Figure 7. Figure 7a shows the TEM image of the Sample 2 sampling location A, and the red box is the sampling location mark. Figure 7b shows the TEM image of the Sample 2 sampling location B, and the orange box is the sampling location mark. Figure  7c shows the TEM image of the Sample 2 sampling location C, and the yellow box is the sampling location mark. Figure 7d shows the HRTEM image of the red box in Figure 7a. The multilayer growth of MoS2 is stacked layer by layer. Figure 7e shows the HRTEM image, where the orange box in Figure 7b is a mixed area, and no MoS2 is observed. Figure  7f is the yellow box in Figure 7c. In the HRTEM image, the mixed zone is shown in the image. Figure 7g is the SAED diagram of Figure 7d, and the miller index is [001]. Here, the lattice arrangement can be seen as multilayer MoS2. Figure 7h is the SAED diagram of Figure 7e. The miller index is [001], and the main composition seen is MoO3. Figure 7i is the SAED diagram of Figure 7f, and the miller index is [001]. The image diffracted from the single-layer MoS2 viewed from this miller index is a lattice point, which may be interpreted as a single-layer MoS2 lattice. Based on the results of the SAED diagram analysis, we can determine from the lattice array that the grown MoS2 material is consistent with the compound calculated using the element ratio. We can interpret that 2-H MoS2 forms under the growth mechanism. Sample 2 is analyzed using the TEM image defined in Figure 7. The sampling position does not pass through the hole. The growth of MoS 2 on the surface is analyzed, as shown in Figure 7. Figure 7a shows the TEM image of the Sample 2 sampling location A, and the red box is the sampling location mark. Figure 7b shows the TEM image of the Sample 2 sampling location B, and the orange box is the sampling location mark. Figure 7c shows the TEM image of the Sample 2 sampling location C, and the yellow box is the sampling location mark. Figure 7d shows the HRTEM image of the red box in Figure 7a. The multilayer growth of MoS 2 is stacked layer by layer. Figure 7e shows the HRTEM image, where the orange box in Figure 7b is a mixed area, and no MoS 2 is observed. Figure 7f is the yellow box in Figure 7c. In the HRTEM image, the mixed zone is shown in the image. Figure 7g is the SAED diagram of Figure 7d, and the miller index is [001]. Here, the lattice arrangement can be seen as multilayer MoS 2 . Figure 7h is the SAED diagram of Figure 7e. The miller index is [001], and the main composition seen is MoO 3 . Figure 7i is the SAED diagram of Figure 7f, and the miller index is [001]. The image diffracted from the single-layer MoS 2 viewed from this miller index is a lattice point, which may be interpreted as a single-layer MoS 2 lattice. Based on the results of the SAED diagram analysis, we can determine from the lattice array that the grown MoS 2 material is consistent with the compound calculated using the element ratio. We can interpret that 2-H MoS 2 forms under the growth mechanism.  Figure 8 is a flow chart of the MoS2 growth process with periodic holes. Figure 8a shows the surface undulation curve of the substrate with periodic holes on ungrown MoS2. The blue line at the top of Figure 8b is the initial stage of growth. A single layer of MoS2 forms when S is dominant, and the purple arrow above represents the direction of Ar. The light brown area in Figure 8c is the mixed region generated when MoO3 is dominated. The composition contains Al2O3, MoO3, and MoS2. Figure 8d shows that when S becomes dominant during the growth period, a single layer or multiple layers of MoS2 are deposited. Although MoS2 grows and overlaps in a curved sheet shape, on the steeper edge of the hole, the sheet-shaped MoS2 grows with the growth side facing upward. In Figure 8e, the brown dashed line indicates the mixed area covered by MoO3 once again during the growth period. Figure 8f is the schematic at the end of the growth. The line segment overlapping on the top represents multiple layers. MoS2 particles overlap with one another in a flake shape. The air flow is thicker after the hole in the direction than before the hole, as seen from the MoS2 on the top. Finally, MoO3 is nearly depleted, and S eventually dominates the final stage of CVD growth. This phenomenon is related to the experimental setting of 0.003 and 1 g of S.  Figure 8 is a flow chart of the MoS 2 growth process with periodic holes. Figure 8a shows the surface undulation curve of the substrate with periodic holes on ungrown MoS 2 . The blue line at the top of Figure 8b is the initial stage of growth. A single layer of MoS 2 forms when S is dominant, and the purple arrow above represents the direction of Ar. The light brown area in Figure 8c is the mixed region generated when MoO 3 is dominated. The composition contains Al 2 O 3 , MoO 3 , and MoS 2 . Figure 8d shows that when S becomes dominant during the growth period, a single layer or multiple layers of MoS 2 are deposited. Although MoS 2 grows and overlaps in a curved sheet shape, on the steeper edge of the hole, the sheet-shaped MoS 2 grows with the growth side facing upward. In Figure 8e, the brown dashed line indicates the mixed area covered by MoO 3 once again during the growth period. Figure 8f is the schematic at the end of the growth. The line segment overlapping on the top represents multiple layers. MoS 2 particles overlap with one another in a flake shape. The air flow is thicker after the hole in the direction than before the hole, as seen from the MoS 2 on the top. Finally, MoO 3 is nearly depleted, and S eventually dominates the final stage of CVD growth. This phenomenon is related to the experimental setting of 0.003 and 1 g of S.  Figure 9 is a schematic of the growth process of MoS2, and Figure 9a is a schematic before growth. The upper layer of the lower sapphire substrate is an amorphous state damaged by laser processing. In Figure 9b, the blue arrow indicates the single layer of MoS2 grown on the substrate surface at the initial stage of growth. In Figure 9c, the blue arrow indicates the coverage of MoO3 at the middle stage of growth, and the green arrow indicates the Al2O3 surface layer at the same time, showing that it mixed at a high temperature to form a mixture zone. The blue arrow in Figure 9d indicates the formation of multiple layers of MoS2, indicating that S gas is greater than MoO3 gas during this growth period. Figure 9e is the schematic of the growth end, and the blue arrow indicates that multiple layers of MoS2 grow to be stacked in sheets. Although in this study, MoS2 is grown on a laser-processed periodic-hole sapphire substrate through CVD, to integrate the developed MoS2 films into practical tools and nanostructures, various patterning and interfacing approaches have been developed. Post-patterning approaches have been successful, indicating wide-ranging applications in current microelectronic techniques, such as FIB milling, photo-and electron-beam lithography, and combinations of metal sputtering processes with selective etching after photolithographically defined masking [51][52][53][54].  Figure 9 is a schematic of the growth process of MoS 2 , and Figure 9a is a schematic before growth. The upper layer of the lower sapphire substrate is an amorphous state damaged by laser processing. In Figure 9b, the blue arrow indicates the single layer of MoS 2 grown on the substrate surface at the initial stage of growth. In Figure 9c, the blue arrow indicates the coverage of MoO 3 at the middle stage of growth, and the green arrow indicates the Al 2 O 3 surface layer at the same time, showing that it mixed at a high temperature to form a mixture zone. The blue arrow in Figure 9d indicates the formation of multiple layers of MoS 2 , indicating that S gas is greater than MoO 3 gas during this growth period. Figure 9e is the schematic of the growth end, and the blue arrow indicates that multiple layers of MoS 2 grow to be stacked in sheets. Although in this study, MoS 2 is grown on a laserprocessed periodic-hole sapphire substrate through CVD, to integrate the developed MoS 2 films into practical tools and nanostructures, various patterning and interfacing approaches have been developed. Post-patterning approaches have been successful, indicating wideranging applications in current microelectronic techniques, such as FIB milling, photo-and electron-beam lithography, and combinations of metal sputtering processes with selective etching after photolithographically defined masking [51][52][53][54].

Conclusions
We used a femtosecond laser to prepare a periodic array of holes with a diameter of about 10 μm on a single-sided polished sapphire substrate. The substrate is grown by CVD of MoS2, and the layer is successfully grown. This technique can be used for growth multiple times, and the results reveal that the size, shape, and number of layers of MoS2 grown each time are different. The possible reason is that the experiment proceeds under atmospheric pressure. Creating a system completely isolated from external interference is impossible, and the influence of external environmental factors on the growth results cannot be predicted. However, other possible causes can be minimized by cleaning the experimental cavity, as well as ensuring minimal differences in parameters and the distance between each substrate installation. The growth mechanism is observed and analyzed using Raman, PL, and AFM analyses to confirm the distribution of the number of generated layers. They are found to possess single-and few-layer positions with excellent photoelectric and semiconductor properties. TEM is used to observe and analyze the cross-sectional image, and the extended function of the TEM is adopted to analyze and convert the SAED image. Subsequently, the growth mechanism is comprehensively analyzed. From the results, we can infer that in a large area of the substrate, MoS2 growth increases and breaks with the direction of Ar until it is concentrated and periodically attaches around the holes, but not at the very end. For the same periodic hole in a small area, MoS2 growth occurs by overlapping pieces, and the number of layers is distributed from the thickest ring around the hole to the single layer at the corner of MoS2. After understanding this growth mechanism, we can select the part of the substrate to be used and the number of layers of MoS2 according to the requirements of the components to be manufactured.

Supplementary Materials:
The following supporting information can be downloaded at: www.mdpi.com/xxx/s1, Figure S1.

Conclusions
We used a femtosecond laser to prepare a periodic array of holes with a diameter of about 10 µm on a single-sided polished sapphire substrate. The substrate is grown by CVD of MoS 2 , and the layer is successfully grown. This technique can be used for growth multiple times, and the results reveal that the size, shape, and number of layers of MoS 2 grown each time are different. The possible reason is that the experiment proceeds under atmospheric pressure. Creating a system completely isolated from external interference is impossible, and the influence of external environmental factors on the growth results cannot be predicted. However, other possible causes can be minimized by cleaning the experimental cavity, as well as ensuring minimal differences in parameters and the distance between each substrate installation. The growth mechanism is observed and analyzed using Raman, PL, and AFM analyses to confirm the distribution of the number of generated layers. They are found to possess single-and few-layer positions with excellent photoelectric and semiconductor properties. TEM is used to observe and analyze the cross-sectional image, and the extended function of the TEM is adopted to analyze and convert the SAED image. Subsequently, the growth mechanism is comprehensively analyzed. From the results, we can infer that in a large area of the substrate, MoS 2 growth increases and breaks with the direction of Ar until it is concentrated and periodically attaches around the holes, but not at the very end. For the same periodic hole in a small area, MoS 2 growth occurs by overlapping pieces, and the number of layers is distributed from the thickest ring around the hole to the single layer at the corner of MoS 2 . After understanding this growth mechanism, we can select the part of the substrate to be used and the number of layers of MoS 2 according to the requirements of the components to be manufactured.

Conflicts of Interest:
The authors declare no conflict of interest.